Tree map study looks at roots of forest change

These historical dioramas in the Harvard Forest Fisher Museum tell the story of the region's forest loss and regrowth. New study shows that pre- and post clearing studies are more similar than you might expect. Photo courtesy of the Harvard Forest.

PETERSHAM — If you could travel back 400 years and inspect the woods that colonial settlers knew, you might be surprised to see how little those woodlands have changed — and yet how much subtle difference there is today from the Northeastern forest of yore.

A new study by Harvard Forest and the Smithsonian Institution, published Wednesday in the journal PLOS ONE, maps trees in a nine-state area and contrasts them with scribbles and dots found on property-boundary documents dating back to 1650, representing more than 350,000 “witness trees.”

The study — comparing relative abundance of certain species in each of 1,300 sampled towns rather than the absolute number of trees — shows an explosion of red maples by more than 20 percent in most areas. There’s also a pronounced loss of oaks and chestnuts.

In western Massachusetts, where the study looked at Greenfield, Montague, Shelburne, Conway, New Salem, Rowe and Shutesbury, as well as Northampton and Athol, there was a dramatic decline of beech, replaced with largely with red maple.

“Red maple survives in shady and sunny conditions, wet and dry conditions,” says lead author Jonathan Thompson, adding that shade-tolerant, slower-growing sugar maples were more dominant in Colonial times. “It’s sort of like the raccoon of tree species. It’s really done well next to humans, it’s done well in disturbed landscapes. It’s everywhere.”

Driving this 10,000-page study, accompanied by about 70 maps, was the massive collection of 400-year-old historical data gathered over three decades by Smithsonian ecologist Charlie Cogbill. The old records he compiled from the public archives of nearly 1,300 towns in nine states from Pennsylvania to Maine were compared to U.S. Forest Service data for those towns.

Harvard Forest Director David Foster, a co-author, says, “It is critical to understand the nature and scale of forest change triggered by human impacts. Charlie’s effort to amass the witness trees provides a serendipitous window into the region’s earliest forests, enabling us to assess the cumulative effect of centuries of land use and climate change.”

Beeches, oaks, and chestnuts show the most pronounced loss — big trouble, Thompson notes, for wildlife that depend on tree nuts for winter survival.

Pine concentrations have shifted more than any other tree type, increasing in some places, decreasing in others. Authors believe that has to do with how quickly it can take over newly cleared land as well as how important it has been for the economy, from building ship masts to boxes to houses.

The region shares a notable forest history: during the 18th and 19th centuries, more than half the forestland was cleared for agriculture and cut for timber. Most farms were eventually abandoned, and during the 20th century, forests returned. Today about 80 percent of the Northeast is forested.

But even two centuries after much of that farmland was abandoned to be overtaken by various successions of trees, the legacy of that activity remains the most powerful factor in determining modern forest composition — more powerful than regional climate, soil conditions and many other factors. If more than half of a town was farmed, the study finds, local forests have probably diverged a good deal from their pre-colonial condition.

Across the board, reports the study, these changes have made modern forests more homogenous and less responsive to small changes in temperature and precipitation. Despite forest clearing, widespread logging, fires, climate change, invasive pests and disease, the Northeast remains the most heavily forested region of the country.

“The overriding theme of this forest region is resilience in the face of multiple impacts,” notes Foster. “This is an important lesson for the future. If we do not replace forests with houses and pavement, they will endure future challenges as well.”

Thompson, a research associate at the Smithsonian Conservation Biology Institute explains, “If you only looked at a tree species list, you’d have the impression that Northeast forests haven’t changed. But once you start mapping the trees, and counting them up, a different picture emerges. In some ways the forest is completely transformed” in terms of which trees are dominate where.

The new study, the first of maybe a dozen that Thompson sees as coming from this large compilation of data, provides an important look at the kind of “massive human-induced disturbance” that went on across the region over such a long period and a large area that it’s been hard to quantify, Thompson says.

Among the future possible studies based on this data, Thompson says, could be a mapping over time of the climactic niche for each tree type, taking into account favored temperatures and precipitation.

“In some ways, it gives me a lot of hope that these lands in New England forests, they want to be forest,” Thompson says of the study. “If you stop mowing the lawn, you’ll get a forest back and that forest will look a lot like forests that have been here for 1,000, 2,000, 3,000 years. In a way it’s remarkable, all the insults and injury we’ve done on this landscape, and really it looks much as it has for a really long time.”